1. Field of the Invention
The present invention relates to a charging apparatus of a printer, and more particularly, to a printer charging apparatus having a simplified voltage generating circuit for charging a photoreceptor medium to a predetermined electric potential.
2. Description of the Related Art
FIG. 1 is a view showing the structure of a typical liquid type electrophotographic color printer. Referring to the drawing, the printer includes a photoreceptor web 11, a discharger 12, a charging apparatus 20, a surface electric potential sensor 13, a plurality of laser scanning units 14, a plurality of developing units 15, a drying unit 16, and a transfer unit 17.
The developing units 15 supply developer for yellow (Y), magenta (M), cyan (C) and black (K) to the photoreceptor web 11 to develop an electrostatic latent image formed by the corresponding laser scanning units 14. The drying unit 16 removes a liquid carrier component remaining on the photoreceptor web 11. The transfer unit 17 transfers a toner image formed on the photoreceptor web 11 to a sheet of paper 18. The discharger 12 removes the electrostatic latent image remaining on the photoreceptor web 11 by emitting light onto the photoreceptor web 11. The surface electric potential sensor 13 detects the electric potential of the photoreceptor web 11.
The charging apparatus 20 charges the photoreceptor web 11 to a predetermined electric potential so that a new electrostatic latent image can be written on the photoreceptor web 11. The charging apparatus 20 includes a first electrode 21 and a second electrode 31 which are separated from each other. The first electrode 21 is typically referred to as a corona electrode and the second electrode 31 is referred to as a grid electrode.
To charge the photoreceptor web 11 in an electromagnetic induction method, the second electrode 31 maintains an electric potential level of the photoreceptor web 11 to be charged or an electric potential slightly greater than the electric potential level of the photoreceptor web 11 to be charged. An electric potential level much greater than that of the second electrode 31 is applied to the first electrode 21. For example, when it is desired that the photoreceptor web 11 be charged to 600 volts, the first electrode 21 is maintained at 5 through 6 kilo-volts while the second electrode 31 is maintained at 600 through 800 volts.
Referring to FIG. 2, the conventional charging apparatus 20 includes a first high voltage generating unit 25 for generating a high voltage to the first electrode 21 and a second high voltage generating unit 35 for generating the second high voltage lower than that of the first electrode 21 to the second electrode 31.
An engine controller 40 outputs information on the voltage level to be generated by the first and second high voltage generating units 25 and 35, to the first and second high voltage generating units 25 and 35. The first high voltage generating unit 25 includes a first PWM (pulse width modulation) controlling portion 22, a first switch 23, and a first transforming portion 24. The second high voltage generating unit 35 includes a second PWM (pulse width modulation) controlling portion 32, a second switch 33, and a second transforming portion 34. Here, voltage generating processes of the first and second high voltage generating units 25 and 35 are the same. The voltage generating process of the first high voltage generating unit 25 is described as follows.
When the engine controller 40 outputs information on a target voltage to be maintained at the first electrode 21 to the first PWM controlling portion 22, the first PWM controlling portion 22 controls the first switch 23 to be turned on/off at a duty set corresponding to the target voltage. Here, the duty signifies a ratio of the time the first switch 23 is turned on during a set period.
When the first switch 23 is turned on/off at a set duty, a voltage increased as much as a coil turn ratio is induced by the first transforming portion 24. The induced voltage is rectified by a capacitor. The voltage rectified by the smoothing capacitor varies depending upon the duty.
The second high voltage generating unit 35 generates a voltage of the second electrode 31 in the same way as that in the first high voltage generating unit 25. However, since the conventional charging apparatus 20 adopts the two high voltage generating units 25 and 35 having different voltage levels to be generated, the structure of the printer is complicated.
To solve the above problem, it is an object of the present invention to provide a charging apparatus of a printer in which the structure of a circuit for charging the photoreceptor web to a predetermined electric potential is made simple.
Accordingly, to achieve the above object, there is provided a charging apparatus of a printer for charging a photoreceptor medium to a predetermined electric potential which includes a first electrode and a second electrode installed to be separated a predetermined distance from the first electrode and to face the photoreceptor medium, in which the charging apparatus comprises a high voltage generating unit for generating a voltage provided to the first electrode, a first varister connected to one end of the second electrode, a second varister having one end connected to the first varister in parallel and the other end connected to a ground terminal, a switching unit connected to the second varister in parallel, and an engine controller for controlling a switching operation of the switching unit to control a voltage maintained at the second electrode.
Also, to achieve the above object, there is provided a charging apparatus of a printer for charging a photoreceptor medium to a predetermined electric potential which includes a first electrode and a second electrode installed to be separated a predetermined distance from the first electrode and to face the photoreceptor medium, in which the charging apparatus comprises a high voltage generating unit for generating a voltage provided to the first electrode, a varister having one end connected to one end of the second electrode, a capacitor having one end connected to the other end of the varister and the other end connected to a ground terminal, a switching unit connected to the capacitor in parallel, and an engine controller for controlling a switching operation of the switching unit to control a voltage maintained at the second electrode.